The semiconductor industry continues to strive for improvements in the speed and performance of semiconductor devices. Strained silicon technology has been shown to enhance carrier mobility in both n and p-channel devices, and thus has been of interest to the semiconductor industry as a means to improve device speed and performance.
One technique for producing strained silicon involves growing silicon (Si) layers on relaxed silicon-germanium (Si—Ge) layers. There is a large mismatch in the cell structure between the Si and Si—Ge layers. This mismatch causes a pseudo-morphic layer of Si on relaxed SiGe to be under biaxial tensile strain. The biaxial strain modifies the band structure and enhances carrier transport in the Si layer. In an electron inversion layer, the subband splitting is larger in strained Si because of the strain-induced band splitting in addition to that provided by quantum confinement. For example, the ground level splitting (E0(d4)-E0(d2)) in a MOS inversion layer at 1 MV/cm transverse field is ˜120 meV for unstrained Si and ˜250 meV for strained Si. The increase in energy splitting reduces inter-valley scattering and enhances NMOSFET mobility, as demonstrated at low (<0.6 MV/cm) and higher (˜1 MV/cm) vertical fields. The scaled transconductance (gm) is also improved due to the reduced density of states and enhanced non-equilibrium transport.
Si has a lattice constant of 5.43095 Å, and Ge has a lattice constant of 5.64613 Å. The lattice constant of SiGe is between the lattice constant of Si and the lattice constant of Ge, and depends on the percentage of Ge in the SiGe layer. FIG. 1 illustrates the lattice constant of a Si1-XGeX substrate for different percentages (X) of Ge. As indicated by FIG. 1, a Si1-XGeX substrate containing about 30% Ge (X≈0.3) has a lattice constant of about 5.50 Å. The biaxial strain of the Si on the SiGe can be calculated as follows:Biaxial_Strain=SiGeLC—SiLC/SiLC,  (1)where the subscript LC represents the lattice constant of the SiGe or Si. Thus, as shown in Equation 2, the Si on the SiGe substrate has a biaxial strain of about 1.28%.Biaxial_Strain≈5.50−5.43/5.43 =1.28%  (2)
FIG. 2 illustrates the mobility enhancement for strained Si for different percentages (X) of Ge in a Si1-XGeX substrate. The mobility enhancement increases as the percentage of Ge in the Si1-XGeX increases, and levels off to around 1.6 when the percentage of Ge is around 22% or larger. Referring to FIG. 1, 22% Ge provides the Si1-XGeX substrate with a lattice constant (SiGeLC) of around 5.485. Using Equation 1, it is determined that the corresponding strain for 22% Ge (the approximate point where the mobility enhancement levels off) is about 1%.
Referring again to FIG. 2, it can be seen that a Si1-XGeX substrate having just under 10% Ge still provides considerable mobility enhancement (1.3). A Si1-XGeX substrate having just under 10% Ge provides the Si1-XGeX substrate with a lattice constant (SiGeLC) of around 5.457. Using Equation 1, it is determined that the corresponding strain is around 0.5%. Thus, it is desirable to achieve a biaxial strain around or greater than 0.5%, and preferably around 1% or greater to obtain the desired enhanced mobility associated with strained Si.
One method for forming the strained Si layer on the relaxed SiGe layer involves epitaxially growing the Si and SiGe layers using an ultra-high vacuum chemical vapor deposition (UHVCVD) process. The UHVCVD process which is a costly and complex process. The Ge content is graded in steps to form a fully relaxed SiGe buffer layer before a thin (˜20 nm) strained Si channel layer is grown. X-ray diffraction analysis can be used to quantify the Ge content and strain relaxation in the SiGe layer. The strain state of the Si channel layer can be confirmed by Raman spectroscopy.
A proposed back end approach for straining silicon applies uniaxial strain to wafers/dies after the integrated circuit process is complete. The dies are thinned to membrane dimensions and then affixed to curved substrates to apply an in-plane, tensile strain after device manufacture.
Research has found that uniaxial strained silicon has advantages over biaxial strained silicon. Less strain is required to obtain an improvement factor if the silicon is strained uniaxially rather than biaxially. Uniaxial strained silicon reduces band gap and in-plane effective mass to improve conduction. Additionally, the work function is altered and contact potentials are reduced.
There is a need in the art to provide improved strained semiconductor films and devices that incorporate the strained films, and to provide improved methods for forming strained semiconductor films.